Method of treating borosilicate glasses



June 16, 1942.

H. P. HOOD ET AL 2,286,275-

METHOD OF TREATING BOROSILICATE GLASSES Filed Sept 10, 1940 2SheetS-Sheet l (ii-\(AN 2o 50 M/MBER or Il0UR$ HEATED INVENTORS. Ina/21sF. #000 By mvo 0)? Manuals/2a A omvx June 16, 1942.

METHOD OF TREATING BOROSILIGA'I'E GLASSES Filed Sept. 10, 1940 2Sheets-Sheet 2 INV TORS. awn/so P 0017 41m n my 5. lYaRDBERG- H. P. HOODEm 2,286,275

Patented ass-16. 194a I'IUNITED T S P EN OFFICE MnTHon F ni gs' m -m m.namion ammo and anti-ohm, Corning, N. Y., assignors to Corning GlassWorks,

Corning, N l -a corporation of New York Application September 19-,iaeogserm No. 356,228

serum. (on. 4a-r9 This applicatiorn is a continuation in part ofourhopending applications Serial No. 744,818, field September 20, 1934,and Serial No. 187,770, filed January 29, 1938. In our pendingapplication, first above mentioned. there is described. a process inwhich glass compositions in the ternary system R2OB2Oa-Si02 are heattreated and thereafter leached with an acid solution to dissolve out themajor portion of the boric oxide and alkali and leave an insolublehighly siliceous residue or skeleton which is submicroscopically porousand permeable to water and which retains Q the original shape of theinitial glass. It is fur-. that shown that the highly siliceous articlethus obtained can be heated slowly to dehydrate it and cansnbsequentlybe revitrified by heating to 900 C. or above to' yield atransparent homogenews article having a composition of approximately 5%1320:, 5% R20, and the balance silic dercertain conditions during theacid leaching step of the above described process a swelling orshrinking of the glass occurs. Such swelling or shrinking establishesstresses in the glass "which may become sufficiently large to causecracking thereof before it has leached through. Such a stresses.practically disappear when leaching has progressed entirely through theglass. r V 5 The primary object of this inventionis to prevent thedevelopment of substantial stress in the slassduring the leaching step.

Another object is to facilitate the production of 'thick walled articlesand were of diverse shapes,

quantity of alumina into the glasses of the prior invention.

A further object is to producesilicate' glass articles inwhich thealkali content of the final product may be reduced to substantiallyzero.

A still further object is to produce homogeneous, transparent articlesof glasses haying expansion coefficients ranging between those of puresilicaand the lowest expansion silicate Y glasses heretofore made.-

Another object is to improve and refine the process and increase itsefilciency.

To these and other ends, the invention comprises various novelimprovements in the method to be more fully set forth anddescribed inthe following specification and claims and illustrated in theaccompanying drawings ,in which; r Fig. 1 is a graph on triangularcoordinates which represents certain glasscompositiona of the ternarysystem R2OB2Oa-Si02 where R is the alkali metal oxide NazO or K20; v

\ Fig. 2 is a plot showing the variation in arbitrary units of straindeveloped in the unleached portion of a glassof specific compositionheat treated at various constant temperatures for di fi ferent lengthsof time.

Fig. 3 is a graph on triangular coordinates rep.-

' resenting a specific field of glass compositions in m the ternary.system NazOB-.-Os-Si0:.

Fig. (is a graph on triangular coordinates representing a specific fieldof glass compositions in the ternary system K:OB2O:-Si02.

' We have discovered that the swelling and is shrinking of'the glass andhence the stresses setup thereby during the leaching step can be com-'pletely initial glass composition from aspecific narrow range ofcompositions preferably containing 20 alumina as an additionalconstituent and heat treating it at 500 C, to 650? C. fora total of fiveminutes to 50 hours. The initial composition to be employed will dependupon the heat treatment intended and, conversely, the specificconditions of heat treatment will depend upon the composition. In otherwords, for a definite heat treatment the swelling or shrinking of theglass is a function only of the initial composition and for a definiteinitial composition it is a function of the temperature and time ofheat-treatment.

For very long heat treatments the time factor is of no importance andswelling or shrinking becomes a function only of initial composition andtemperature. Heat treatment is effective not h 5 only during the timethat the glass is held at a Another object is to introduce a sub ta ti ldefinite temperature but'is also eilective during the time that theglass is cooling withinthe I above recited' range. Therefore time. ofheat' treatment as referred to herein, unless otherwise -specified,includes not only the time during glass isf J-also' affected by thetemperature and strength of the leaching acid, although to a minordegree onl'lhfas will appear.

In describing the invention itis convenient to fix all variables withthe exception. of one in or- 5o der to considerthe' permissiblevariations in the latter within'the range of which little or no strainwill be produced during.leaching. Accordingly, the initial glasscompositions will-be. considered for example with respect to a heattreatment at a constant temperature of 550 0,

controlled or eliminated by selecting the from the line.

II III IV Hal can:

Although the'line N within the limits of experimental error representscompositions of glasses in which no stress will be developed under theabove recited conditions, variations in these conditions will produce aslight deviation of the line, as will later appear. In. general,glasses, the

compositions of which are represented by points at the right of theline, will swell when subjected to the heat treating and leachingconditions noted above and glasses represented by points slightly at theleft of the line will shrink under those conditions, as is indicated inFig. 1. Glasses too far to the left will not leach. Swelling orshrinking on leaching will become more pronounced as the composition isfurther removed During leaching, swelling will cause tensional stress inthe unleached portion of the glass and shrinking will cause tensionalstress in the leached layer. Since breakage is caused by tensionalstress rather than by compression and since the leached layer isinherently weaker than the unleached portion and further since surfaceflaws may cause further weakening, shrinking is more objectionable thanswelling. A further disadvantage of shrinking is that, for a givensilica content, it is in general" accompanied by slower leaching.Compositions in this system, which are suitable for use under the aboverecited fixed conditions, are defined as those containing 55% to 70%$102, minus .1X)% NazQX being theexcess of silica over 55%, and thebalance being B203.

In the above recited range of compositions, K may besubstituted whollyor in part for NazO, but in that case it is necessary also to decreasesomewhat the ratio of alkali to boric oxide by weight. An example ofsuch substitution is the following composition calculated from the batchin which N8-2O is completely substituted by K20;

Sine K'ZO In Fig. 1 a line K defines all compositions in the in theglass during the leaching step. The same relationships apply to the lineK as were described in the consideration of the: line N. Compositions inthis system which are suitable foruse under the fixed conditionsaredefined as,

those containing 55% to S102, (8.5 minus .-1X)% m0, x being the" excessof silica over 55%, and the balance being B203.

Considering the two lines N and K, it will be noted that in each thesilica contents vary from 55% to 70%, but in line K the alkali oxidevaries from 7% to 8.5%, whereas in line N the alkali oxide contentvaries from 8.5% to 10%, the balance of the composition in each casebeing boric oxide.

or both of the alkalies may be, expressed as com The compositionscontaining either one prising 55% to 70% S102 and (10 minus 1.5Y minus.1X)% R20, where R20 is the total K20 and Nazo, Y is the ratio finalglass compositions for which there is evidence that they are more stableagainst devitrification than the prior glasses. As pointed out in theprior copending application first above mentioned, the introduction ofalumina reduces the rate at which leaching can take place, but it hasbeen found that this rate can be restored by a sufficient reduction ofthe silica content' wlthin the limits of the field. In order to maintainzero strain the percentage of NazO must be decreased. The amount bywhich the percentage of Na=0 must be decreased for any given addition ofA120: within the range of .1-4% A1203 has been found to be proportionalroughly to the square of the A1203 content and the proportionalityfactor is about .17. As the A1203 content is increased up the 4% thelower limit of the S10: content is decreased in the proportion 1.25%SiOa for each per cent of A1201, and witha content of 4% A1203 the S10:may be as low as 50%. Therefore, glasses in the quaternary system Na2oA1=o=-Bto -s1o2, I 1 which will develop substantially no strain whenheat treated and leached under the above recited fixed conditions, areconfined to a range of compositions which may be defined bythe follow-.

ing expression: (55 minus 1.25Z) to 70% SiOa, .1. to 4% A1203, (10 minus.12! minus .17Z)% NazO and the remainder B203, where Z is the percentageof A120: and X is the excess of 310:,

"that is to say, the lines N and K represent glasses in whichsubstantially no stress will be developed when they are heat treated ata constant temperature of 550 C. for 20 hours, and are subsequentlyleached in 3 normal acid at when the time and/or temperature of heattreatment are altered within limits, the points representing glasses,which will develop substantially no stress, will not fall exactly uponthese lines but will be slightly removed therefrom and will fall at oneside or the other of the lines as the case maybe.

nection with the .affect the amount of stress developed during leachingand hence must be considered in contreatment.

An example of the relationship between time and temperature of heattreatment is shown in the curves of Fig. 2 which represent the resultsobtained on leaching a series of samples '1 mm.

A positive strain denotes a swelling of the glass on leaching and anegative strain denotes a shrinkage. The point on the strain axisthrough which the curves pass when extended represents the resultobtained for this glass withl no heat treatment'other than that acquiredduring fabrication. With the exception of the curve for 600 it will benoted that, as the duration of heat treatment increases, the leachingstrain increases algebraically toward a maximum, which is different foreach temperature chosen. As the employed. with sumciently high initialtemtime or duration or the heat peratures and proper cooling ratessubh-proce dure will also produce the desired result. For

example, substantially no strain will result when the glass referred toabove is heated for'three hours at 580 C. and is cooled through thecritical range at aboutper hour.

The variations in amount of strain caused by variations in time andtemperature of heat treatment, which are described above for a glass ofspecific composition, are substantially the same in type for allcompositions on and in the im- ,mediate neighborhood of the lines N andK shown in Fig. 1 and for compositions in the above defined quaternarysystem. Times and temperatures of heat. treatment which cause positivestrain on leaching may be compensated and substantially zero strain mayresult, despite such conditions, by shifting the composition of theglass to the left or in the direction of increasing tension as indicatedin Fig. 1. Conversely timesand temperatures of heat treatment whichcause negative strain-on leaching may be compensated by shifting thecompositions to the right. In other words, conditions of heat treatmentwhich would cause swelling on leaching require an increase inthe R20 orA1203 or both for a given B203 content in orderto prevent temperatureoiheat treatment is decreasedthe maximum strain which canbe attainedbecomes greater. However, with decreasing temperature the rate of changein the structure of the, glass becomes slower, partly as a result of theincreased viscosity. Hence the highest compression, that is, swelling,on leaching is obtained through the use of the lowest temperature ofheat treatment, provided a suflicient length of time is employed.Temperatures lower than 500 C. require too long, atime to be practical.for the purpose of the present invention.

It will further be noted that as the tempera-, ture of heat treatment ofthis glass is increased the attainable maximum positive strain becomesless and for temperatures in the neighborhood of 600 C. only negativestrain can be attained. On account of crazing with this glass the pointsfor the curve for 600 C. are approximations. Forthis particularcomposition aconstant temperature heat-treatmentat 600 C. would not bepractical. However, with suitable compositions still higher temperaturesup to about 650 C. may be employed.

Further inspection of the curves of Fig. 2 will show that this glass canbe leached withpositions of the above defined glasses may be thedevelopment of strain 'on leaching, and conditions of heat treatmentwhich would cause shrinking on leaching. require a decrease.

From the above it 'will now be apparent that in the case of unavoidablevariations in composition of the above defined glasses whereby, with agiven heat treatment, strain would be developed during leaching, suchvariations in composition may be compensated by suitable variation intime and/or temperature of heat treatment and thus leaching may becarried out without the development of substantial strain.

Such control constitutes one of the chief advantages of the inventionbecause, despite the most rigid control of batch composition, ap-

preciable variations in'glass composition occur {from one melt to thenext and in a continuous tank the composition of the glass may vary ap-'preciably from day to day. By the construction of curves as shown inFig. 2, appropriate changes maybe made in the time and/or temperature ofheat treatment and the efiiciency of production may thus be maintainedat a high level.

The extent to which variations in the comcompensated by'varying theconditions of heat treatment or, in other words, the extent of thefields of -compositions which can be leached without the development ofsubstantial strain out the development of objectionable strain byemploying any conditions of temperature and time represented by theseand similar curves for intermediate temperatures where they intersect orclosely approach the zero ordinate.

For example, a heat treatment for about 20 hours or longer'at about 575C. will produce r is cooled slowly the eflect is as though. succesbysuitable variation and controlof the conditions of heat treatment areillustrated for the ternary systems in Figs. 3 and 4 by the areas withinthe parallelograms designated N1 and'Ki respectively.

In Fig. 3 it will be seen that compositions failing within the area ,Nlcontain from to SiOz and from (10.5 minus .1X)% to (8.5 minus .1X)% NazOwhere X is the excess of silica over 55%, the balance being B203.

in Fig. 4 the compositions falling within the area K1 contain from 55%to 70% S102 and from (9 minus .11!) to (7.0 minus .12!) K20 where xis'the excess of silica over 55%, the balance 4 B2034 4 I Consideringthe two areas Ni and K1 it will .be noted that in each the silicacontents -vary sively lower constant temperatures had been from 55%to70%, but in areaNi the alkali oxide K R20 and X is the excess Si02over 55%, and containing boric oxide.

For the quaternary. system the extent of the field of compositions isexpressed as (55 minus 1.25Z) to 70% Si02, .l to 4% A1203, (10.5 minus.lX minus .17Z to (8.5 minus .lX minus .17Z NazO, and the remainderB203, where Z is the percentage of A120: and X is the excess $102 over55%, X being negative when the SiOz is less than 55%. As an example, acomposition-in the field thus defined which we have found to beparticularly suitable comprises Per cent SiO2 62.7 N320 6.6 A1203 3.5B203 26.9

The third variable factor in the control of swelling and shrinking, ashereinbefore pointed out, is the leaching condition or principally thetemperature and concentration of the leaching acid. This factor hasrelatively minor importance with regard to total swelling. A decrease intemperature of leaching will tend to increase the compressional stressin the leached layer slightly if the glass swells on leaching and willtend to decrease the tensional stress slightly if the glass shrinks onleaching. Increasing the concentration of acid will tend slightly todecrease the swelling on leaching. The choice of leaching conditions isgoverned by practical considerations and substantially zero strain forthe chosen conditions is effected entirely by proper choice of glasscomposition and heat treatment. Thus for practical purposes thetemperature of the. leaching acid should be as high as possible, inorder to insure the most rapid leaching. Increase of the acidconcentration within limits also would increase the rate of leaching,but unfortunately this criterion cannot govern the choice ofconcentration because with high concentrations there is a tendency forthe leached layer to spall or split ofi. This condition is aggravated bythe accumulation of saltsand boric acid in the leaching bath resultingfrom the reaction. Such condition becomes more critical as the thicknessof-the ware is increased; A concentration of about 1 normal givessatisfactory results.

In practicing the invention a composition falling within one of theabove defined fields is melted in the usual manner and fabricated'intoware. The heat treatment conditions suitable for the glass selected areascertained by determining the relation between the amount oj'straindeveloped on leaching and the time and tem- .perature of heat treatmentas described abpve.

The articles are then heat treated under the'conditions thus ascertainedand are leached preferably under the-condition of the highesttemperature and concentration of the leaching bath that is compatiblewith the thickness of the article and are subsequently washed withwater. The resulting glasses are porous in structure and may bevitrified by heating to a temperature in the neighborhood-of 900C. to1200 C.

By means of the hereinbefore described process, .we are able to producearticles of glass containing over 95% silica, from0.02% to 0.25% alkalioxide and from 2% to 6% boric oxide. We have also succeeded in producingglasses consisting of over 94% SiOz, 0.02% to 0.25% N820, 0.3% to 2%A1203, and 2% to 6% B201. As an example of such a glass the followingcomposition is given: 95.5% SiOz, 0.05% M1120, 0.8%

A1203; and 3.7% B203.

We claim:

1. The process which includes melting a glass containing 55% to 70% Si02and (10.5 minus 1.5Y minus .lX)% to (8.5 minus 1".5Y minus .lX) R20, andcontaining B203, where R20 is the total N920 and K20, Y is the ratio 2.The process which includes melting a glass containing (55 minus 1.252)to 70% Si02, 0.1%

to 4% A1203, (10.5 minus .lX minus .17Z to (8.5 minus .lX minus .17ZNa20 and containing B203, where Z is the percentage of A1203 and X isthe excess of Si02 over 55%, X being negative when the Si02 is less than55%, fabricating the glass into. a fixed shape, heating the article at atemperature between 500 C. and 650 C. for five minutes to 50 hours, andleaching the heat treated article in an acid solution.

3. The process which includes melting a glass containing 55% to' 70%Si02 and from (10.5 minus .lX)% to (8.5 minus .lX)% Na20, and containingB203, where X is the excess of S102 over 55%, fabricating the glass intoa fixed shape, heating the article at a temperature between 500 C. and650 C. for five minutes to 50 hours, and leaching the heat treatedarticle in an acid solution. r

4. The process which includes melting a glass containing 55% to 70% Si02and from (9 minus .lX)% to (7.0 minus .lX)% K20 and-containing B203;where X is the excess of 5402 over 55%, fabricating the glass into afixedshape, heating the article at a temperature between 500 C. and 650C. for five minutes to 50 hours, and leaching the heat treated articlein an acid solution.

5. The process which includes melting a glass comprising approximately62.7% $102, 6.6% Na20, 3.5% A: and 26.9% 13203, fabricating the glassinto a fixed shape, heating the article at about 580 C. for about 3hours, coolingit at the rate of about 25.degrees per hour tobelow 550C., then cooling it more rapidly to room temperature, and leaching theheat treated article in an acid solution.

" HARRISON P. HOOD.

MARTIN E. NORDBERG'. I

